Photopolymerized copolymer films

ABSTRACT

Copolymer films are provided which are made by the surface photopolymerization of vaporous mixture of a diene such as butadiene and a vinyl monomer, for example acrylonitrile. The copolymer films exhibit valuable insulating properties and have a dielectric constant over the range of from about 2.65 to about 5.9.

C United States Patent 1 1 5 0 Wnght 145] Jan. 18, 1972 [5 1PHOTOPOLYMERIZED COPOLYMER 156] References Cited FILMS UNITED STATESPATENTS [72] f Archibald wrigh" smemmdy' 3,271,180 9/1966 White..117/93.31 [73] Asaignee: General Electric Company 3,012,950 12/1961Anderson ..204/ 159.22 X

3, 58,8 1 1 2 O 9. 3 Filed: 9, 69 0 99 0/ 96 Yankoet al 2 4/15 2 X 21APPL 3 3 372 Primary Examiner-Alfred L. Leavitt Assistant Examiner-.1.H. Newsome Related U.S. Application Data Attorney-Richard R. Brainard,Paul A. Frank, Joseph T. Cohen, Charles T. Watts, William A. Teoli,Frank L. Neu- [63] Contlnuatlon-ln-part of Ser. No. 530,950, Mar. 1,hausenoscar B waddenand Joseph B Fol.man

1966, abandoned.

[52] 1.1.8. Cl. ..l17/93.3l, 204/159.22, 117/132 CB, [57] ABSTRACT ll7/161 UH, 1 17/161 UN, 260/823, 260/83 7 Cupolymer films are providedwhich are made by the surface [51] Int. Cl. ..B44d 1/50, C08f 1/18photopolymerization of vaporous mixture of a diene such as [58] Field ofSearch ..ll7/93.31,93.3;260/92.3, butadiene and a vinyl monomer, forexample acrylonitrile.

The copolymer films exhibit valuable insulating properties and have adielectric constant over the range of from about 2.65 to about 5.9.

5 Claims, 3 Drawing Figures PHGTUPULYMERHZED COPOILYMER lFllLMS Thisapplication is a continuation-in-part of my copending application Ser.No. 530,950, filed Mar. 1, 1966, now abandoned and assigned to the sameassignee as the present invention.

This invention relates to photopolymerized films, coatings, and productsincluding such films or coatings, and to methods of forming such films,coatings, and products, and more par I ticularly to continuous films,coatings, and products formed by simultaneous ultraviolet surfacephotopolymerization of two gaseous materials, and to methods of formingsuch films, coatings, and products.

Thin films, which can be configurationally deposited are desirable for awide variety of applications. It is further desirable that such thinfilms and coatings be adhesive to a substrate, and continuous thereon.The present invention is directed to improved thin films, coatings andproducts having such films or coatings thereon which exhibit the abovedesirable characteristics and to methods of forming such films,coatings, and products having such films or coatings. The thin films andcoatings of the present invention are formed by simultaneous ultravioletsurface photopolymerization of a gaseous diene and of a gaseous vinylmonomer. Such vinyl monomers include, for example, ethylene, methylmethacrylate, styrene, and acrylonitrile.

In addition to being configurationally deposited, continuous andadhesive, the films and coatings formed in accordance with my inventionexhibit good chemical resistance, have preselected dielectric constants,are pinhole-free, and exhibit good temperature stability. These filmsand coatings are useful for a wide variety of applications includingcapacitor dielec trics, covering layers for various metallic andnonmetallic sub strates, cryogenic device insulation, insulation formicroelectric devices, as a primer or as insulation on electricallyconductive wire, and for corrosion protection.

It is an object of my invention to provide a method of forming acontinuous film by simultaneous ultraviolet surface photopolymerizationof two gaseous materials.

It is another object of my invention to provide a method of forming in apredetermined pattern such a continuous film.

It is another object of my invention to provide a method of forming acontinuous film by simultaneous ultraviolet surface photopolymerizationof two gaseous materials in which the substrate is cooled duringphotopolymerization to increase the rate of film formation.

It is a further object of my invention to provide a method of forming acontinuous film on a substrate by simultaneous ul travioletphotopolymerization of two gaseous materials thereby forming a productor composite article.

It is a still further object of my invention to provide a method offorming a continuous coating on a substrate by simultaneous ultravioletsurface photopolymerization of two gaseous materials and removingsubsequently the substrate by chemical etching.

In accordance with my invention, a continuous film can be formed bysimultaneous ultraviolet surface photopolymerization of a gaseous dieneand a gaseous vinyl monomer.

These and various other objects, features and advantages of theinvention will be better understood from the following description takenin connection with the accompanying drawing in which:

FIG. 1 is a perspective view partially in section of an apparatus forforming films, coatings and products in accordance with my invention; 7

FIG. 2 is an enlarged side elevational view partially in section of aportion of the apparatus shown in FIG. 1; and

FIG. 3 is a sectional view of a substrate with a thin film thereonformed in accordance with my invention.

In FIG. 1 of the drawing, apparatus is shown generally at for fon'ningfilms, coatings, and products having such films or coatings thereon inaccordance with my invention. A base or support surface (not shown) isprovided on which is mounted an L-shupcd bracket 11 to support anenclosure or chamber 12 having a flange 13 at its open end. A quartztube 14 is bonded adjacent at its open end by a suitable metal-ceramicseal to a metal cylinder 15 having a flange 16 at its opposite end.Flange 16 is readily threaded to and unthreaded from flange 13 ofenclosure 12 by means of a plurality of threaded fasteners 17. A vacuumpump 18 is connected by a line 19 to enclosure 12 to evacuate enclosure12 and associated quartz tube 141. A control valve 20 is provided inevacuation line 19. An inlet line 21 is connected at one end toenclosure 12 and at its other end to two sources (not shown) ofdifferent materials to be supplied in gaseous state to tube 14. Acontrol valve 22 is provided in line 21 to control the supply ofmaterial to enclosure 12 and tube 14. If desired, one gaseous materialis supplied through inlet line 21 while the second gaseous material issupplied through an inlet line 21 with a. control valve 22.

A support block 23 of a material such as copper is shown positionedwithin tube 14. Block 23 has a U-shaped metal tube 24 imbedded therein,two ends 25 and 26 of which extend through cylinder 15, flanges 16 and13, enclosure 12 and through the wall of enclosure 12. Tube 24circulates a cooling medium such as ethanol to block 23 and positionsthe block. The ends 25 and 26 of tube '24 are connected to a heatexchanger or to other cooling equipment. A substrate support 27 is shownpositioned on support block 23. Substrate support 27 comprises, forexample a 1 inch X 3 inch glass microscope slide on the upper surface ofwhich is a 0.25 micron aluminum film substrate 28. A stainless steellight mask 29, which is shown as the same size as the substrate support27, is shown also with three slots 30 therethrough to provide formationof predetermined patterned thin films or coatings on the aluminum filmsubstrate. An ultraviolet light 31, which is normally provided with areflector (not shown), is shown outside and spaced above quartz tube 14and supported in any suitable manner. Such a light source providesultraviolet light in a region of about 2,000 to 3,500 angstroms, whichis directed by the reflector (not shown) toward the upper surface ofaluminum film 28. For example, a Hanovia 700-watt lamp with a reflectorwill provide this particular light region. A metal enclosure with adoor, which is not shown, is positioned around the above apparatusduring its operation.

In FIG. 2 of the drawing, an enlarged side elevational view is shown ofsupport block 23 which was described above in connection with FIG. 1 ofthe drawing. Block 23 has a U-shaped tube 24 imbedded therein, the twoends 25 and 26 of which circulate a cooling medium to and from block 23,respectively. Substrate support 27 and light mask 29 are shown partiallyin section to disclose more clearly the aluminum film substrate 28thereon. While three slots 30 are described for light mask 29, a singleslot or a plurality of slots either connected or disconnected may beemployed. Masks are also usable which have different configurationalpatterns.

In FIG. 3 of the drawing, there is shown a glass substrate support 27with a 0.25-micron thick aluminum film substrate 28 thereon. Acontinuous film 32 is shown adhering finnly to the upper surface of thealuminum film 28 in accordance with the method of my invention using theapparatus shown in FIG. 1.

I have discovered unexpectedly that a continuous film could be formedwhich comprises simultaneously photopolymerizing a gaseous diene and agaseous vinyl monomer. Such vinyl monomers include ethylene, methylmethacrylate, styrene,

and aerylonitrile on the surface of a substrate member with ultravioletlight having an effective wavelength preferably in the range of 2,000 to3,500 angstroms at a vapor pressure for the gaseous material in therange of from 3 to 4 millimeters of mercury. I have also found thatthese continuous films are pinhole-free. I have discovered that furtheradvantages can be derived by cooling the substrate during the formationof the film thereon thereby increasing the rate of film fonnation. Ihave found further that subsequent to the formation of the above type ofcontinuous film formed on the substrate, the substrate could be removed,for instance, by chemical etching with hydrochloric acid or hydrofluoricacid, thereby providing an unsupported body of the film.

I found further that the simultaneous ultraviolet surfacephotopolymerization or copolymerization of the above gaseous materialsprovides a great advantage in that a vinyl monomer embodying particularselected properties can be incorporated into a continuous film ofconsiderable structural strength produced as a result of crosslinking ofdiene units present in the chain. For example, styrene provides anaromatic group resulting in relative stability while methyl methacrylateprovides a polar function.

The dielectric constant of the continuous film depends upon the monomeremployed to produce the film. In this manner, you are restricted tofixed values if you use different monomers. I found that intermediatevalues for the dielectric constant were secured when I mixed these dieneand vinyl monomers. Thus, I can preselect the dielectric constant forthe resulting film. In microelectric apparatus this could be a distinctadvantage.

Thin films, which are formed from styrene alone, are not pinhole-free.However, when I formed films from a gaseous styrene and a gaseousbutadiene composition, these films were pinhole-free. Whileacrylonitrile provides a pinhole-free thin film with a high dielectricconstant, I found that I increased greatly the rate of deposition for anacrylonitrile-butadiene mixture over the rate for either monomer alone.

In an illustrative operation of the apparatus shown in FIG. 1 of thedrawing, a substrate support 27 in the form of a I inch X 3 inch glassmicroscope slide with a 0.25-micron thick aluminum film substrate 28thereon was positioned on copper support block 23. A stainless steellight mask 29 of dimensions 1 inch X 3 inches with three slots thereinwas placed on the upper surface of the aluminum film substrate 28thereby covering film substrate 28 except for slots 30. Quartz tube 14was then attached by its flange 16 to flange 13 to enclosure 12 by meansof threaded fasteners 17. Vacuum pump 18 was started and pumped down thechamber defined by tube 14, cylinder 15, and enclosure 12 to a pressureof about 1 micron of mercury. Valve 20 was then closed. A diene, such asfor example, 1,3-butadiene, 1,5-hexadiene or 2,4-hexadiene, was suppliedfrom a liquid source (not shown) through line 21 in a gaseous state toenclosure 12 whereby it was fed into the intenor of quartz tube 14. Avinyl monomer was supplied from a liquid source (not shown) through line21' in a gaseous state to enclosure 12 whereby it was fed into theinterior of quartz tube 14. Each of the above materials is initiallyretained in its liquid state by maintaining its temperature below roomtemperature which is accomplished by employing a cooling bathsurrounding the liquid materials. The liquids are all maintained at avapor pressure in the range of 3 to 4 millimeters of mercury by thetemperature of the cooling bath whereby its introduction from the sourceto the inlet line is in a gaseous state. Ultraviolet lamp 31 waspositioned above quartz tube 14 and spaced approximately 2 inches fromthe upper surface of aluminum film 28. The lamp has an effective wavelength in the range of 2,000 to 3,500 angstroms.

The monomers were introduced into quartz tube 14 and the pressure rose.A metal hood (not shown) is positioned around apparatus since anultraviolet light source is used. Lamp 31 is turned on. After a periodof time, lamp 32 was shut off, monomer valve 22 was closed, and thesystem was pumped down to about 2 microns pressure to remove allbyproducts. The metal hood was removed and the vacuum was then broken.Tube 14 was cooled to room temperature and disconnected by unthreadingfasteners 17 which held its associated flange 16 to flange 13. Aftertube 14 was removed, metal light mask 29 was removed and substratesupport 27 was picked up and examined. A continuous film had beenfom'ied on aluminum film substrate 28 which was pinhole-free.

While it is stated above in the operation of the apparatus of FIG. 1,that an aluminum film substrate was employed for the formation thereonof a continuous film formed from the gaseous material, many othermetallic and nonmetallic substrates in various forms and configurationscan be employed in the process. For example, such a film is formed onmetallic substrates including lead, niobium, copper, gold, steel, iron,brass, and aluminum. Various nonmetallic materials are employed such asglass, quartz, mica, carbon, diamonds and borazon.

Examples of films, coatings and products including such films andcoatings embodying my invention and methods of making such films andcoatings and products including such films and coatings in accordancewith my invention are set forth below:

EXAMPLE l Apparatus was set up in accordance with FIG. 1 of the drawing.A substrate support, a microscope glass slide l inch X 3 inches, whichwas provided with a 0.25-micron thick aluminum film substrate thereon,was positioned on the copper support block. A stainless steel light mask1 inch 3inches and having three slots therein was placed on the surfaceof the aluminum substrate. The quartz tube was positioned around thesupport block by threading its flange to the flange of the enclosure towhich the gaseous materials supply lines and vacuum pump were connected.An ultraviolet light source, in the form of a Hanovia 700-watt lamp witha reflector was positioned above the quartz tube and spaced about 2inches from the upper surface of the aluminum film substrate. The systemwas pumped down to a pressure of 1 micron of mercury and the controlvalve was closed. Butadiene and styrene were maintained at their sources(not shown) in liquid form by positioning in separate cooling baths.Butadiene was introduced in the gaseous state at an initial pressure of4 millimeters of mercury into the quartz tube by opening the controlvalve in the respective inlet line. The control valve was then closed.Styrene was then introduced in the gaseous state at a partial pressureof 4 millimeters of mercury into the quartz tube by opening the controlvalve in the respective control valve. Both of these monomers wereintroduced in initial equimolar concentrations. A metal hood waspositioned around the apparatus. The lamp, which had an effective wavelength in the range of from 2,000 to 3,500 angstroms, was turned on.Butadiene and styrene in gaseous state were present in the quartz tubeunder the above light for a period of 5 minutes. In this operation, afilm was formed on the aluminum film substrate by simultaneousultraviolet surface photopolymerization of gaseous butadiene andstyrene.

While it is not shown in the drawing, a plurality of thermocouples wasprovided to measure the temperature of the substrate and of the surfaceof the evaporated aluminum film to provide temperature information.Cooling means for the substrate as shown in FIG. 1 of the drawing anddescribed above were employed thereby maintaining'an average temperatureof the substrate at 73 C. The process was concluded by discontinuing thesupply of gaseous butadiene and styrene, turning off the ultravioletlight source, removing the hood, opening the vacuum pump control valve,and pumping down the interior of enclosure 12 and tube 14 to a pressureof about 1 micron to remove gaseous material and any byproductstherefrom. The vacuum was then broken and the quartz tube was removed byunthreading its flange from the enclosure flange. The light mask wasremoved and the aluminum film on the glass substrate was examined.Visual examination disclosed three separate thin films, each of whichwas continuous. The film was measured by capacitance andinterferrometric techniques and found to have an average thickness of200 angstroms. Thus, the growth rate was 40 angstroms per minute. Thefilm was further tested and its breakdown strength was determined to be5.8 volts DC at 220 angstroms. The dielectric constant for this film was2.7, while the dielectric constant for a film from butadiene is 2.65 andfor a film from styrene is 2.8. It will be appreciated by those skilledin the art that a change in the concentration varies the dielectricconstant.

Thus, a product was obtained from this example which comprised a glassbase with an aluminum film substrate thereon on which a continuous,pinhole-free thin film adhered to the upper surface of the substrate.

EXAMPLES 2-3 Styrene films having thicknesses of 530 Angstroms and Inthe following examples, the same apparatus, substrate, fi zgi'respicntvely i madedby UV g g materials and procedures were followed asin example 1. p 2; t 0 S mm: e Table 1 sets forth below the examplenumber, the time of film me l us a e y examp e Styrene formation inminutes, the average substrate temperature in phoiopolymenzed at aPressure of torr whllfi degrees Centigrade, the average fil thickness inangstroms, taming a substrate temperature below 00 C., to provide for athe growth rate of the film in angstroms per minute, and th Satisfactorygrowth rate. in addition, butadrene-styrene breakdown strength of thefilm in volts direct current. Each of copolymer films having thicknessesof about 200 Angstroms the films was continuous. and 1,040 Angstromsrespectively also were made utilizing TABLE 1 Average Average filmGrowth Breakdown Example Time substrate thickrate, strength, DielectricNumber (min.) temp.,C. nose, A. A./min. volts TLC. constant 2 31 1,04069 14.0 at1,040 A". 2.7 3 30 89 2, 510 84 15.0 at 2,510 A--. 2. 7

EXAMPLES 46 substantially the same conditions from an equal molarmixture of styrenebutadiene.

In the following examples, the same apparatus, b trat The various filmswere then evaluated as dielectric layers by and procedures were followedas in example l. Butadiene and the same procedure. A mobile mercury dropelectrode was acrylonitrile were employed in initial equimolarconcentraemployed to determine breakdown voltage. Table 1 shows thetions. The dielectric constant for a film from acrylonitrile is resultsobtained, where Styrene indicates the styrene film, 5.9. Table 2, whichhas similar headings to table 1, sets forth Copolymer indicates thebutadiene-styrene copolymer and below these examples. Each of thesefilms was continuous. A indicates thickness in Angstroms:

TABLE 2' Average Average film Growth Breakdown Example Time substratethickrate, strength, Dielectric Number (min.) temp.,C. ness,A. AJmin.volts D.C. constant EXAMPLES 7-9 i TABLE 4 In the following examples,the same apparatus, substrate, and procedures are followed as inexample 1. The monomers A Breakdown strength were introduced in initialequimolar concentrations. The dielectric constants for methylmethacrylate, 1,5-hexadiene,

d 2 4-hexadiene are a roximatel 3 3 and 4 res ectivel 530 shorted an y p3,560 Shortcd in spots Table 3, Wi'llCh has similar headings to tables 1and 2, set forth Copolymer 200 r 10- volts/cm. below these examples.Each of the films is continuous. The 1,040 lXl0volts/cm. dielectricconstants of examples 7, 8 and 9 are approximately 2.8, and 3,lfiSPBCtiVCi). resistance less than 3Xl0 ohms overall TABLE 3 EXAMPLE llin accordance with the procedure of example 10, there it Time 1 52: 55were made butadlene films having thicknesses of 3,100 Ang- Example noMonomers Minutes Tcmp.C. stroms and 7,500 Angstroms respectively, and abutadiene acrylonitrile copolymer film having a thickness of 4,300 Ang-7 butadiene, methyl 15 70 stroms. The films were made by the UV surfacemethacoflale photopolymerization of the corresponding aliphatically un-8 f 'i saturated material, utilizing a pressure of about 4 torr and aacrylomtnle o 9 Mmuadicne, 15 go substrate surface temperature belowabout 100 C. The

styrene capacitance-s of the butadiene films and thebutadieneacrylonitrile copolymer films were measured in accordance withthe mercury drop technique. All of the films were found A n G h i 'if 55to be pinhole-free and useful as dielectric materials. The varivcrage 1mrowt rate 0 tagc Example no. Thickness A. Ajminute v u DC ous films werethen sub ected to an accelerated atmospheric moisture test by exposingthem to steam arising from bOlilIlg water in a beaker at a point about 3inches above the top of a 7 1,500 100 so 1000 700 5 beaker. After a l0second exposure, the various films were 9 13. I10 then measured forcapacitance following the same technique I employed before beingsubjected to the accelerated moisture test.

Table 5 below shows the results obtained, where Butadiene" indicates thebutadiene films, Copolymer" indicates EX M LE 1Q thebutadiene'acrylonitrile films, Capacitance" is expressed in Farads, andBefore" and After" is expressed with respect to exposure to steam.

TABLE Capacitance A. Before After Butadienc BJOO 7.8Xl0" Shorted 7,5003.2)(10 Shorted copolymer 4,300 ILSXIO'IO ILOXIO derived fromingredients used in making the copolymer films.

While other modifications of the invention and variations of methodwhich may be employed within the scope of the invention have not beendescribed, the invention is intended to include such as may be embracedin the following claims.

I claim:

1. A continuous pinhole-free film having a thickness of from about 200Angstroms to 13,500 Angstroms, said film being the ultraviolet surfacephotopolymerized product of a mixture of butadiene and a gaseous vinylmonomer from the group consisting of styrene and acrylonitrile.

2. A composite of the film of claim I and a metallic substrate.

3. A composite of the film of claim 1 and a nonmetallic substrate.

4. A continuous pinhole-free film in accordance with claim 1 where thevinyl monomer is styrene.

5. A continuous pinhole-free film in accordance with claim 1 where thevinyl monomer is acrylonitrile.

2. A composite of the film of claim 1 and a metallic substrate.
 3. Acomposite of the film of claim 1 and a nonmetallic substrate.
 4. Acontinuous pinhole-free film in accordance with claim 1 where the vinylmonomer is styrene.
 5. A continuous pinhole-free film in accordance withclaim 1 where the vinyl monomer is acrylonitrile.